A plant is considered healthy when it can carry out its physiological functions, such as cell division, differentiation, development, photosynthesis, absorption and translocation of water and nutrients from the soil, metabolism, reproduction, and storage of food supplies, without disruption. When plant functions are disturbed by pathogens, the plants become diseased. Disease can be defined as the malfunctioning of plant host cells and tissues caused by continuous irritation by a pathogenic agent. A disease involves abnormal changes in the form, physiology, or behavior of the plant.
Plant pathogens cause disease by weakening the plant by absorbing food from the plant cells, secreting toxins, enzymes, or growth regulating substances that disturb or kill the plant cells, or block the transport of food nutrients or water in the plant. The roots, stems, leaves, flowers, or fruits can be infected. The affected cells and tissues are weakened or destroyed, and cannot perform normal physiological functions, resulting in reduction of plant growth or death, and reducing crop quality or yield. The major causes of plant diseases are bacteria, mycoplasmas, viruses, nematodes, and fungi. Fugal species from a variety of genera affect plants, including Fusarium, Pythium, Phytophthora, Verticillium, Rhizoctonia, Macrophonmina, Thielaviopsis, Sclerotinia, and numerous others. Plant disease caused by fungi include pre- and post-emergence seedling damping-off, hypocotyl rots, root rots, crown rots, vascular wilt, and other symptoms. Nematodes harmful to plants include nematode species form the genera Meloidogyne, Heterodera, Ditylenchus, and Pratylencus. Plant diseases caused by nematodes include root galls, root rot, lesions, "stubby" root, stunting, and other rots and wilts. Some nematodes (e.g., Trichodorus, Lonoidorus, xipenema) can serve as vectors for virus diseases in a number of plants including Prunus, grape, tobacco, and tomato.
Plant pathogenic bacteria cause a variety of plant disease symptoms. About 80 species of bacteria (e.g., Pseudomonas viridiflava, Xanthomonas campestris pv. asclepiadas, Xyellafastidiosa, Acidovorax albilineans, and Acidovorax avenae sspl citrulli) cause disease in plants, including fruit rot, galls, wilts, blight, and leaf spots. As bacteria multiply quickly, controlling them early in the disease process is critical. Copper and streptomycin compounds are the only chemical compounds currently available for the control of bacterial diseases.
The response of plants to microbial attack involves de novo synthesis of an array of proteins designed to restrict the growth of the pathogen. These proteins include hydroxyproline-rich glycoproteins, proteinase inhibitors, enzymes for the synthesis of phyoalexins, enzymes contributing to the reinforcement of cell walls, and certain hydrolytic enzymes.
Plant defenses can also be activated by elicitors derived from microbial cell walls and culture fluids. In dicotyledonous plants, extensive studies have shown that microbial attack or elicitor treatments induces the transcription of a battery of genes encoding proteins involved in the defense response, as part of a massive switch in the overall pattern of gene expression. In contrast, little is known about the inducible defenses in monocotyledonous plants.
Genetic engineering of plants, which entails the isolation and manipulation of genetic material, e.g., DNA or RNA, and the subsequent introduction of that material into a plant or plant cells, has changed plant breeding and agriculture considerably over recent years. Increased crop food values, higher yields, feed value, reduced production costs, pest resistance, stress tolerance, drought resistance, the production of pharmaceuticals, chemicals and biological molecules as well as other beneficial traits are all potentially achievable through genetic engineering techniques. Genetic engineering techniques supplying the genes involved in pathogen resistance have the potential to substantially affect crop production.